The present invention relates to a device and a method for the capacitive analysis of a moving strand-like, preferably textile, test material according to the preamble of the independent claims. It is preferably, but not exclusively, used in the off-line measurement of mass unevenness of yarn, roving or sliver, as carried out on textile laboratory test instruments.
EP-0′924′518 A1 discloses a device for the capacitive measurement of the properties of a textile product such as a sliver, roving or yarn. For the purpose of better understanding of this state of the art, a number of aspects of EP-0′924′518 A1 which are in connection with the present invention will be illustrated in the enclosed FIGS. 1 and 2.
The illustration of FIG. 1 corresponds to that of FIG. 13 of EP-0′924′518 A1, and shows five carrier plates 101.1-101.5 which form four lead-through openings or measuring gaps 102.1-102.4. The test material 9 is guided through one of the measuring gaps 102.2. Since the carrier plates 101.1-101.5 are situated substantially parallel next to one another, the test material 9 can only be inserted precisely into one measuring gap 102.2 and can be moved through said gap along its longitudinal axis. The measuring gaps 102.1-102.4 each comprise one electrode 103.1, 103.2 of a measuring capacitor and one respective electrode (not shown in FIG. 1) of a compensation capacitor in or on its two side walls between which the product 9 can be guided. Disturbing influences such as local changes in humidity or deformations in the capacitor geometry can be compensated effectively by means of the compensation capacitor whose capacitance is as large as that of the measuring capacitor. Electrical lines 104.1, 104.2 connect the measuring and compensation capacitors to a measuring circuit (not shown in FIG. 1). The measuring gaps 102.1-102.4 have different widths, so that the test material 9, depending on its cross-section, can be measured in a measuring gap 102.2 with suitable width. Principally, it is desired to select the measuring gap width in such a way that on the one hand the test material 9 can be guided through the measuring gap 102.2 without touching its walls, but that on the other hand the measuring gap 102.2 is not much wider than the cross-section of the test material 9. The larger the fraction of the measuring gap 102.2 that is filled out by the test material, the higher the sensitivity of the device relating to the properties of the test material 9 changing during the measurement, e.g. mass unevenness. The widths of the measuring gap 102.1-102.4 can lie in the range of between 0.1 mm and 10 mm, for example.
FIG. 2, which substantially corresponds to FIG. 2 of EP-0′924′518 A1, shows a measuring circuit 1′ configured as a half measuring bridge with four measuring capacitors 2.1′-2.4′ and four associated compensation capacitors 3.1′-3.4′. The measuring capacitors 2.1′-2.4′ are connected in parallel with respect to each other and form a first half bridge branch 20′, whereas the compensation capacitors 3.1′-3.4′, which are also connected in parallel with respect to each other, form a second half bridge branch 30′. The two half bridge branches 20′, 30′ are connected in series with respect to each other, and an output signal is tapped between them on a line 5′. Equally large alternating voltages in anti-phase are applied to the two half bridge branches 20′, 30′ by alternating voltage generators 4.1′, 4.2′. When the half measuring bridge 1′ has been calibrated and no test material 9′ is situated in one of the measuring capacitors 2.1′-2.4′, the two half bridge branches 20′, 30′ have equally large total capacitances; the output signal is therefore zero. If on the other hand a test material 9′ is inserted into a measuring gap of a measuring capacitor 2.2′, it influences the capacitance of the measuring capacitor 2.2′. The change in capacitance of the respective measuring capacitor 2.2′ that is generated in this manner disturbs the equilibrium between the measuring capacitors 2.1′-2.4′ and the compensation capacitors 3.1′-3.4′, so that an alternating voltage is obtained as an output signal whose amplitude is proportional to the mass of the test material 9′ between the measuring electrodes. This output signal is processed in a signal processing unit 6′, e.g. it is amplified, filtered and/or converted, and evaluated as a measure for the mass per unit of length of the test material 9′.
Devices are known from WO-2005/033697 A1 or U.S. Pat. No. 3,731,069 A which comprise several identical lead-through openings with identical measuring capacitors. In the respective measuring methods, several yarns are tested simultaneously, i.e., one yarn in each lead-through opening. In this process, the capacitive measuring signals of the measuring capacitors are supplied serially or sequentially to a common electronic evaluation system. U.S. Pat. No. 6,369,588 B1 discloses a device and a method for the capacitive analysis of a fabric. The fabric moved along its longitudinal direction is scanned over its entire width by several capacitive sensors. Signals of the capacitive sensors are supplied serially to a common microprocessor for evaluation.
WO-2008/128363 A1 discloses a measuring capacitor for a yarn which comprises two measurement electrodes which are arranged behind one another in the direction of movement of the yarn. The effective length of the measuring field can be varied in that short yarn defects are detected with only one of the two measurement electrodes and long yarn defects with both together. The two measurement electrodes can also be used for measuring the speed or length by means of runtime correlation.